Richard Hannay undertakes a perilous mission to pursue the elusive "Greenmantle". His success or failure could change the outcome of World War I.

It is November, 1915. Major-General Sir Richard Hannay, KCB, OBE, DSO, Legion of Honour, is called in to investigate rumours of an uprising in the Muslim world, and undertakes a perilous journey through enemy territory to meet his friend Sandy in Constantinople. Once there, he and his friends must thwart the Germans' plans to use religion to help them win the war, climaxing at the battle of Erzurum.